The present invention relates to data transfer in a time division multiplexing environment. More particularly, the present invention relates to using associative memories to implement a limited switching scheme that generates and receives messages in a time division multiplexing environment.
Improvements in communications networks have led to robust designs that support multiple devices connected to a single bus. Typically the supported devices consist of microprocessors, memory devices, digital to analog converters, analog to digital converters, digital signal processing devices, printers, modems, and universal serial bus hubs. The communication between the multiple devices is governed by the type of network and the band width of the individual devices. A conventional communication scheme that reduces connection costs while increasing portability between devices is a shared serial interface.
In a shared serial interface, each device coupled to the shared interface transmits/receives data on the shared serial bus. Typically, the shared serial bus includes a data line, a clock, and control signals. In the prior art, a full-switch in conjunction with a time division multiplexing (xe2x80x9cTDMxe2x80x9d) scheme is used to schedule the transmission of data across the shared serial bus. TDM is the division of a time interval, also denoted as a frame, into a number of equal sub-intervals called time slots. The beginning of each time frame is typically identified by a start of frame (xe2x80x9cSOFxe2x80x9d) signal transmitted along the shared serial bus. The TDM scheme allocates one or more time slots to each device coupled to the shared serial bus. Accordingly, the device can only transmit/receive data on an assigned time slot(s). The allocation of time slots and transmit commands is maintained by the full-switch.
FIG. 1 illustrates a prior art full-switch TDM system. In particular, system 100 includes input 110, memory devices (RAM 120 and 150), output 130, and control 140. Input 110 transfers the serial data from each transmitting devicexe2x80x94typically referred to as an incoming TDM streamxe2x80x94to system 100. Output 130 transmits serial data from system 100xe2x80x94typically referred to as an outgoing TDM streamxe2x80x94to each receiving device.
Control 140 switches the data stream on input 110 to create a different output stream on output 130, thus creating a switching mechanism between different devices coupled to input 110 and output 130. Specifically, control 140 sequentially writes data for each time slot into an address of RAM 120. For example, data received in the first time slot is written into the first address of RAM 120 and data received in the second time slot is written into the second address of RAM 120. As previously described, each time slot corresponds to a different device transmitting data. Thus, in the present example, device one writes data to the first memory address of RAM 120 and device two writes data to the second memory address of RAM 120. After writing the input data stream to RAM 120, control 140 uses memory 150 to generate an output data stream.
As illustrated in FIG. 1, control 140 is also connected to memory 150. Each address of memory 150 corresponds to a specific time slot on the outgoing stream 130. Each address of memory 150 also contains connection information that determines which address of RAM 120 should be transmitted on output 130. For example, the third memory address of memory 150 stores data having a value of xe2x80x98thirty-two.xe2x80x99 Accordingly, during time slot 3, the data stored in the thirty-second memory address of RAM 120, from a previous time frame, is transmitted on output 130. Thus, resulting in the transfer of data from the device transmitting on slot thirty-two to a specific device that receives data in the third time slot. The specific device that receives data during the third time slot is pre-determined by system 100.
A full-switch TDM system provides a data transfer mechanism that switches data between all time slots in any given order. Thus, the full-switch TDM system allows any two devices coupled to the shared serial line to transfer data. The full-switch TDM system, however, results in numerous disadvantages.
One disadvantage of using a full-switch TDM system results from the size requirements of RAM 120 and memory 150. In a thirty-two time slot TDM scheme, both RAM 120 and memory 150 require thirty-two rows of memory cells. The large memory requirements result in system 100 using a significant area. The large memory requirements also result in an increased cost for system 100.
Another disadvantage of using a full-switch TDM system results from a network system where all the time slots in the TDM scheme are not required. For example, in a thirty-two time slot TDM scheme used in a system with only four devices only the first four memory addresses of RAM 120 and memory 150 are used during data transfer. In this network environment, memory locations five through thirty-two of RAM 120 and memory 150 remain unused.
A method for employing an associative memory to implement a message passing switch is disclosed. The method comprises the step of receiving data in a time slot. The method also comprises the step of examining an interrupt register to determine if the data is a message. Additionally, the method comprises the step of storing the data in a memory location and transferring the data to an external device.